1. Field of the Invention
The present invention relates to an optical fiber with cured polymeric coating.
More particularly, the present invention relates to an optical fiber with at least one protective coating layer having a reduced attenuation of the transmitted signal.
Moreover, the present invention relates to an optical fiber with at least one protective coating layer obtained by curing a radiation curable composition comprising at least one ethylenically unsaturated polyurethane and at least one polyfunctional reactive diluent monomer and also to a radiation curable composition used therein.
Moreover, the present invention also relates to a method for controlling the attenuation losses caused by microbending on the signal transmitted by an optical fiber.
2. Description of the Related Art
Optical fibers commonly consist of a glass portion (typically with a diameter of about 125 μm), inside which the transmitted optical signal is confined, and of a coating, typically polymeric, arranged around the glass portion for substantially protective purposes. This protective coating typically comprises a first coating layer positioned directly onto the glass surface, known as the “primary coating” or “primary” for short, typically having a thickness of between about 25 μm and about 35 μm. In turn, this primary coating is generally covered with a second coating layer, known as the “secondary coating” or “secondary” for short, typically having a thickness of between about 10 μm and about 30 μm.
These polymeric coatings may be obtained from compositions comprising oligomers and monomers that are generally crosslinked by means of UV irradiation in the presence of a suitable photo-initiator. The two coating layers described above differ, inter alia, in terms of the modulus of elasticity value of the crosslinked material. As a matter of fact, one problem presented by the use of coating layers which are adhered to the glass surface of the optical fiber is caused by the difference in response to change in temperature between the glass and the coating layer which contributes to microbending attenution of the fiber, especially when very low temperatures are encountered. To minimize this problem, coating layer possessing a very low modulus of elasticity value are selected to provide the above mentioned primary coating. Consequently, in order to provide the desired low modulus of elasticity value in the primary coating, one must sacrifice desired hardness and thoughness in the coating layer which contact the glass, so as the above mentioned secondary coating has to be applied on the top of said primary coating. The combination of said two layers of coating ensures adequate mechanical protection for the optical fiber.
The optical fiber thus composed usually has a total diameter of about 250 μm. However, for particular applications, this total diameter may also be smaller; in this case, a coating layer of reduced thickness is generally applied.
However, the necessity of using two coating layers having different characteristics may present some drawbacks. For example, problems due to the adhesion between the primary and the secondary coatings may arise: it is therefore necessary to select polymeric materials which are compatible among themselves but which have different modulus of elasticity values in order to both avoid microbending and to obtain an adequate mechanical protection.
In order to overcome said drawbacks, some efforts have been made in the prior art to obtain coating compositions which may be used as a single coating layer for optical fibers.
For example, U.S. Pat. No. 4,806,574 discloses an ultraviolet curable liquid coating composition which, when cured with ultraviolet light in the presence of an appropriate photoinitiator, provides a coating adapted for the coating of optical glass fiber. This coating composition comprises as the essential component, an acrylate-terminated polyurethane oligomer based on a polyfunctional core which is at least trifunctional and which supports one branch for each functionality in the core. According to the assertions made in the patent, said coating composition may be used as a topcoat as well as a coating directly applied onto the glass surface of the fiber in order to provide low tensile modulus at the low service temperatures which may be encountered so as to resist microbending. In one embodiment, said cured coating composition has a tensile modulus measured at +25° C. of 6,410 psi (about 44 MPa) and a tensile modulus measured at −40° C. of 96,971 psi (about 669 MPa).
U.S. Pat. No. 4,682,850 discloses an optical fiber having a core and an outer cladding. The cladding is coated with only a single ultraviolet-cured material having tensile modulus in the range of about 1,000 to about 10,000 psi (about 7 MPa to about 70 MPa). Preferably, the modulus is about 7,800 psi (about 53.8 MPa) measured at +25° C. and the material has a Shore A hardness of about 70 to about 75. According to the assertion made in the patent, said single coating satisfactorily protects the optical fiber, is easily applied to the fiber and minimizes microbending losses over a wide temperature range.
Other documents, such as, for example, U.S. Pat. Nos. 4,690,501, 4,690,502, 4,798,852, 4,932,750, disclose optical fiber coating compositions adapted either as primary coating or single coating, generally mentioning that these are suitable for minimizing microbending. Moreover, no specific value of the tensile modulus of the coating compositions measured either at +25° C. or at −40° C. is given in said documents.
In spite of the efforts to obtain suitable single coating layers, no satisfactory solution has however yet been found. In particular, whilst the above mentioned documents stress the need to avoid microbending at the low operating temperatures (i.e. −40° C.), most of these documents give no details about the mechanical properties of the used coating layers at such low temperatures. As a matter of fact, only U.S. Pat. No. 4,806,574 above cited discloses an example of a coating layer having a tensile modulus measured at −40° C. of about 668 MPa. Applicant has however observed that this value is still too high to significantly avoid the microbending phenomena.
In addition, Applicant has observed that the value of tensile modulus of said coating layer from the room temperature (+25° C.) to the low operating temperatures of −40° C., undergoes to an excessive variation, which variation in turn determines an excessive and uncontrolled variation of the microbending attenuation on the optical fiber.
Applicant has observed the behaviour of both (A) a commercial single coating DeSolite® 3471-3-7 (DSM) and of (B) a single coating obtained by mixing 63% of a commercial primary coating DeSolite® 3471-1-129 (DSM) and 37% of a commercial secondary coating DeSolite® 3471-2-136 (DSM) in order to have a modulus of elasticity value measured at +25° C. of about 60 MPa as suggested in U.S. Pat. No. 4,682,850 above cited: the two single coatings show however an excessively high increase of the modulus of elasticity value measured at −40° C. as showed in the enclosed FIG. 3 (in the abscissa is reported the temperature value (T) in ° C. as in the ordinate is reported the modulus of elasticity value (E′) in MPa). Said modulus of elasticity value is tensile modulus and is measured using a DMTA apparatus (Dynamic Mechanical Thermal Analyser from Reometrics Inc.) operating as will be better described below.
The Applicant has further observed that the tensile modulus of the coating layer should be controlled over a broader range than the one indicated by the prior art (from −40° C. to +25° C.). As a matter of fact, under normal operative conditions, an optical fiber may be easily subjected to temperatures of about +40° C. and, in particular cases, up to about 60° C. Thus, it is important that the value of the tensile modulus of the coating layer remains sufficiently high also at such high operating temperatures in order to suitably protect the glass portion of the optical fiber.